This application is based on application No. 99-22766 filed in the Korean Industrial Property Office on Jun. 17, 1999, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same and, more particularly, to a positive active material for a rechargeable lithium battery which exhibits good cycle life characteristics.
(b) Description of the Related Art
Rechargeable lithium batteries employ materials into or from which lithium ions are intercalated or deintercalated as negative and positive active materials.
For the negative active material in a rechargeable lithium battery, metallic lithium has been used. However, lithium metal has good reactivity toward electrolyte and deposits to form a dendric layer which induces short circuit between the negative and positive active material. There are at least two disadvantageous effects that arise from the reaction of lithium with electrolyte: the exothermic liberation of heat and the formation of passive films on lithium has been shown to be one reason for the loss of capacity of lithium cells on repeated cycling.
The problem of lithium reactivity toward the electrolyte is addressed by replacing lithium metal with carbon-based materials. With the use of carbon-based active materials, the potential safety problem present in metallic lithium-based batteries can be prevented while achieving a relatively higher energy density as well as the reasonable shelf life.
For the positive active material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are inserted or extracted are used. Typical examples include LiCoO2, LiMn2O4, LiNiO2, LiNi1xe2x88x92xCoxO2(0 less than X less than 1) or LiMnO2. LiCoO2 is widely used as it has an electrical conductivity of about 10xe2x88x922 to 1 S/cm at room temperature and high battery voltage, but has poor safety characteristics during high rate charges and discharges. LiNiO2 has a high charge capacity, but is difficultly produced. Manganese-based material such as LiMn2O4 or LiMnO2, etc. has low capacities but is easiest to prepare, is less expensive than the other materials and has environmentally friendly characteristics.
However, when a battery using the manganese-based materials such as LiMn2O4 is charged and discharged for a long time, particularly, at a high temperature, side reaction between an electrolyte and the manganese-based material occurs at the surface of the manganese-based material. It is believed that H2O reacts with LiPF6 in an electrolyte to generate strong acid HF which attacks to Mn presented in a surface of the manganese-based active material and the attacked Mn is eluted into the electrolyte. The eluted Mn is dissolved in the electrolyte and the active material is disintegrated. The side reaction seriously deteriorates the cycle life characteristics of the battery.
The attempt to solve this problem has been by synthesizing a material including 1 or more equivalent of Li or partially substituting oxygen with F in a spinel manganese-based material. However, the methods do not effectively improve the cycle life characteristics of the battery, particularly, at high temperature.
It is an object of the present invention to provide a positive active material for a rechargeable lithium battery which exhibits good cycle life characteristics, particularly at high temperature.
It is still another object to provide a method of preparing the positive active material for the rechargeable lithium battery.
These and other objects may be achieved by the positive active material for a rechargeable lithium battery including a manganese-based compound selected from the group consisting of compounds represented by formulas 1 to 6. The surface of the active material is coated with metal oxide.
LixMn1xe2x88x92xMxe2x80x2xA2xe2x80x83xe2x80x83(1)
LixMn1xe2x88x92xMxe2x80x2xO2xe2x88x92zAzxe2x80x83xe2x80x83(2)
xe2x80x83LixMn1xe2x88x92xxe2x88x92yMxe2x80x2xMxe2x80x3yA2xe2x80x83xe2x80x83(3)
LixMn2xe2x88x92xMxe2x80x2xO4xe2x80x83xe2x80x83(4)
LixMn2xe2x88x92xMxe2x80x2xO4xe2x88x92zAzxe2x80x83xe2x80x83(5)
LixMn2xe2x88x92xxe2x88x92yMxe2x80x2xMxe2x80x3yA4xe2x80x83xe2x80x83(6)
where, 0.01xe2x89xa6xxe2x89xa60.1, 0.01xe2x89xa6yxe2x89xa60.1, 0.01xe2x89xa6zxe2x89xa60.5, Mxe2x80x2 is at least one semi-metal selected from Si, B, Ti, Ga, Ge or Al, Mxe2x80x3 is at least one transition metal or lanthanide metal selected from Al, Cr, Co, Mg, La, Ce, Sr or V, and A is selected from O, F, S or P.
The present invention further includes a method of preparing a positive active material for a rechargeable lithium battery. In this method, a manganese-based material selected from the group consisting of formulas 1 to 6 is obtained. The material is coated with a metallic alkoxide solution or metal aqueous solution. The coated powder is then heat-treated such that the coated powder is converted into a metal oxide-coated powder.